For a long time, scientists have been searching for an answer as to how galaxy clusters regulate the number of stars they create. Given that the amount of interstellar gas used to create the stellar giants exists in such abundance, this theoretically allows for the creation of many times the current number of stars. A team of researchers from MIT, Columbia University and Michigan State University believe they have found the answer.
The standard process for star formation takes place when intercluster gas cools extremely quickly to the point where it is able to condense and collapse upon itself, forming a young star in the process. However this process is relatively rare, as the gigantic clusters are often too hot to allow many stars to be created.
"The amount of fuel for star formation outpaces the amount of stars 10 times, so these clusters should be really star-rich," states Michael McDonald, a Hubble fellow at MIT's Kavli Institute for Astrophysics and Space Research. "You really need some mechanism to prevent gas from cooling, otherwise the universe would have 10 times as many stars."
The question then becomes, how do galaxies maintain these extreme temperatures? It is well-established that all galaxies in the known universe fall broadly into two distinct categories – cool core clusters and non-cool core clusters. Cool-core clusters are ordinarily older than their non-cool cousins, having had time for their temperature to drop enough to create a limited number of stars. Non-cool clusters however are still incredibly hot, with temperatures in the range of hundreds of millions of degrees Celsius.
By taking into account a cluster's mass, density, radius and temperature, the team are able to identify the threshold at which interstellar gas begins to cool fast enough for the star creation process to trigger. Depending on whether the temperature of the galaxy is significantly above or below the threshold, one of two reheating mechanisms works to prevent prolific star formation.
If the cluster's temperature is significantly above the threshold, star birth will be severely limited by a mechanism known as thermal conduction. In its most basic form, conduction is the transfer of energy – in this case heat – across the entirety of a body. In the context of a galactic cluster, this means that even if a region of the cluster manages to cool enough to dip below the threshold allowing it to create stars, the intense heat of the surrounding gas would quickly heat the cooler pocket.
Conversely, if the temperature of a cluster were to fall significantly below the threshold at which star creation becomes possible, it is still prevented from making use of the vast majority of interstellar gas due to a mechanism called precipitation-driven feedback. This phenomenon occurs when some of the newly-cooled gas falls back into one of the supermassive black holes lying at the center of the galaxies that make up the cluster, spewing out re-heated material that serves to raise the temperature above the threshold once more.
The theory has been tested using data collected from NASA's Chandra X-Ray Observatory and the South Pole Telescope, and the findings seem to support the theory. Looking to the future, it will be further tested to see if the team's predictions apply to the reheating mechanisms at work within the individual galaxies that make up the clusters.
A paper outlining the team's work has been published in the online journal Nature.
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